In order to cause a light source such as a semiconductor laser to generate very short light pulses, a light source driving device has been proposed in the art in which a comb generator, or avalanche element, or two step recovery diodes are used to drive a light source with a short voltage pulse.
The light source driving device with the comb generator, as shown in FIG. 7, comprises a resonance circuit 50 and a step recovery diode 51 so that, with respect to a sine wave input voltage V.sub.a, a very short pulse voltage V.sub.b having a pulse width of the order of 100 to 200 pico-seconds is outputted to drive the light source. The light source driving device using the avalanche element outputs a short pulse voltage on its switching characteristic, quick avalanche breakdown, to drive the light source.
A light source driving device having two step recovery diodes is as shown in FIG. 8. That is, the device comprises two step recovery diodes 52 and 53. The rise time and fall time of a pulse like input voltage V.sub.c is reduced, or the bias current of the step recovery diode 53 is increased by means of a constant current source 55, to produce a very short pulse voltage V.sub.d of the order of several hundreds of pico-seconds to drive the light source.
The light source driving device with the comb generator employs the resonance circuit 50, as was described above. Therefore, in the device it is necessary to match the frequency of the input voltage V.sub.a with the resonance frequency of the resonance circuit 50. For instance, when the resonance frequency is 50 MHz, it is impossible to set the very short pulse voltage V.sub.b to other than 50 MHz. The light source driving device using the avalanche element operates on avalanche breakdown, and therefore the repetitive frequency of the short pulse voltage is of the order the maximum of which is 100 KHz because of factors such as avalanche recovery time and so on.
In the light source driving device having two step recovery diodes 52 and 53 as shown in FIG. 8, the input voltage V.sub.c is applied through a capacitor 56 to the step recovery diode 52, and therefore the waveform of a voltage V.sub.g applied to the step recovery diode 52 changes with the frequency of the input voltage V.sub.c. In other words, with respect to the input voltage V.sub.c as shown in part (a) of FIG. 9, the voltage V.sub.g across the diode 52 is raised from ground, zero volts, to as much as .DELTA.V.sub.g, which corresponds to the increment in frequency of the input voltage V.sub.c as shown in part (b) of FIG. 9. Therefore, it is not possible to change the frequency of the input voltage V.sub.c to provide a very short pulse voltage V.sub.d having a given repetitive frequency.
In the light source driving device shown in FIG. 8, the very short pulse voltage V.sub.d is about half (1/2) of the input voltage V.sub.c. Therefore, in order to output a very short pulse voltage V.sub.d high in magnitude it is necessary to receive a high input voltage. Furthermore, the use of two constant current sources 54 and 55 makes the circuit complex.
The conventional light source driving device thus constructed cannot output a very short pulse voltage having a given repetitive frequency in a wide range of frequencies without having a changing waveform. Accordingly, it is impossible to provide single very short light pulses or very short light pulses of repetitive frequencies up to several tens of mega-Hertz (MHz) with their waveforms unaffected by the change in frequency.